U.S. patent number 4,209,804 [Application Number 06/008,133] was granted by the patent office on 1980-06-24 for record carrier containing information in an optically readable radiation reflecting information structure.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Jan G. Dil.
United States Patent |
4,209,804 |
Dil |
June 24, 1980 |
Record carrier containing information in an optically readable
radiation reflecting information structure
Abstract
A record carrier is described having an optically readable
radiation-reflecting information structure, comprising information
areas arranged in information tracks, which areas are spaced from
each other by intermediate areas, the information areas having
oblique walls. It is demonstrated that a suitable information
signal and a suitable positional error signal are obtained if the
angle of inclination of the walls of the information area lies
between 65.degree. and 85.degree. and the phase depth of the
information areas lies between 95.degree. and 140.degree..
Inventors: |
Dil; Jan G. (Eindhoven,
NL) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
19831738 |
Appl.
No.: |
06/008,133 |
Filed: |
January 31, 1979 |
Foreign Application Priority Data
|
|
|
|
|
Oct 19, 1978 [NL] |
|
|
7810462 |
|
Current U.S.
Class: |
369/275.3;
235/487; 369/105; 369/275.4; 369/109.01; G9B/7.139; G9B/7.097;
G9B/7.029 |
Current CPC
Class: |
G11B
7/12 (20130101); G11B 7/24076 (20130101); G11B
7/24079 (20130101) |
Current International
Class: |
G11B
7/24 (20060101); G11B 7/007 (20060101); G11B
7/12 (20060101); H04N 005/76 (); G06K 019/06 () |
Field of
Search: |
;358/127,128 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Simplified Diffraction Theory of the Video Disk," Applied Optics,
vol. 17, No. 13, Jul. 1978, pp. 2037-2042..
|
Primary Examiner: Cook; Daryl W.
Attorney, Agent or Firm: Briody; Thomas A. Streeter; William
J. Cohen; Simon L.
Claims
What is claimed is:
1. A record carrier containing information in an optically readable
radiation reflecting information structure comprising information
areas, which are arranged in information tracks, which are spaced
from each other in the track direction by intermediate areas, and
which have a phase depth which is substantially constant over the
entire record carrier, characterized in that the cross-section,
transverse to the track direction, of the information areas is
substantially V-shaped, that the phase depth of the information
areas has one value between 100.degree. and 125.degree., and that
the angle of inclination between the walls of the information areas
and the normal to the record carrier is substantially constant and
has a value between 65.degree. and 85.degree..
2. A record carrier as claimed in claim 1, characterized in that
the phase depth is approximately 110.degree..
3. A record carrier as claimed in claim 2, adapted to be read with
a read beam produced by a helium-neon gas laser and having a
wavelength of approximately 633 nm, in which record carrier the
width, transverse to the track direction, of the information areas
is approximately 625 nm, characterized in that the angle of
inclination of the information areas is approximately
78.degree..
4. A record carrier as claimed in claim 2, which is adapted to be
read with a read beam produced by an AlGaAs diode laser with a
wavelength in the range from 780 to 860 nm and with a direction of
polarization parallel to the track direction, in which record
carrier the width, transverse to the track direction, of the
information areas is approximately 625 nm, characterized in that
the angle of inclination of the information areas is approximately
73.degree..
5. A record carrier as claimed in claim 1, characterized in that
between first information tracks containing information areas with
a phase depth between 100.degree. and 110.degree. second
information tracks are formed which contain information areas whose
phase depth is approximately 180.degree..
6. A record carrier as claimed in claim 1, characterized in that
consecutive track portions within a track differ from each other in
that they comprise information areas with a phase depth between
100.degree. and 110.degree. and information areas with a phase
depth of approximately 180.degree. respectively.
7. A record carrier as claimed in claim 1, in which record carrier
in predetermined track portions, information can be written by a
user, characterized in that information is contained only in sector
addresses in which addresses of associated unrecorded track
portions, which contain a material which is inscribable with
radiation, area provided, the information areas in the sector
addresses having a phase depth between 100.degree. and
110.degree..
8. A record carrier as claimed in claim 7, characterized in that
the unrecorded track portions have a phase depth of approximately
100.degree..
Description
The invention relates to a record carrier containing information in
an optically readable radiation-reflecting information structure
comprising information areas, which are arranged in information
tracks, which are spaced from each other in the track direction by
intermediate areas, and which have a phase depth which is
substantially constant over the entire record carrier.
An optical record carrier, especially as a medium for the
dissemination of a (color) television program, is inter alia
described in the article: "Simplified diffraction theory of the
video disk" in: "Applied Optics," Vol. 17, No. 13, July 1978, pages
2037-2042. During reading the information structure is illuminated
with a read beam which by means of an objective system is focussed
on the information structure to a read spot of the order of
magnitude of the information areas. In the path of the read beam
which has been modulated by the information structure there is
arranged a radiation sensitive information detection system, whose
output signal varies in accordance with the portion of the
information structure being read instantaneously. As is described
in the said article, the information structure may be regarded as a
diffraction grating which splits the read beam into a plurality of
spectral orders, to which orders a specific phase and amplitude may
be attributed. For reading the information mainly the zero order
subbeam and the first order subbeams, which are diffracted in the
track direction, area of interest. The first order subbeams
interfere with the zero order subbeam at the location of the
information detection system. Use can be made of so-called
"push-pull" detection i.e. the difference between the output
signals of two detectors is determined. These detectors are then
disposed in the far field of the information structure, behind each
other viewed in the track direction. The difference signal of the
two detectors is then determined by the information being read
instantaneously.
According to the afore-mentioned article the difference signal, or
information signal, is optimum, i.e. the signal has a maximum
modulation depth, if the phase difference between a beam portion
coming from an information area and the beam portion coming from
the surrounding of said area is 90.degree.. This means that for a
record carrier which is read in transmission the optical depth of
the information areas should be 1/4.lambda.eff, where .lambda.eff
is the wavelength at the location of the information structure. The
information structure is preferably a reflecting structure. In that
case the information areas should have an optical depth equal to
1/8.lambda.eff for an optimum reading by push-pull detection. It is
then assumed that the walls of the information areas are
perpendicular, or in other words that the angle of inclination of
these walls is 0.degree.. The angle of inclination is to be
understood to mean the acute angle between the walls and a normal
to the plane of the information structure.
For some time the applicant has been using the concept of "phase
depth" for the information structure. This phase depth is defined
as the difference between the zero spectral order phase and one of
the first spectral order phases, if the center of the read spot
coincides with the center of an information area. In general this
phase depth differs from the phase difference defined in the
afore-mentioned article. Only if the phase difference is
180.degree. at the angle of inclination of the information areas is
0.degree., the phase difference approximates the phase depth. For
an angle of inclination of 0.degree. a phase difference of
90.degree. does not correspond to a phase depth of 90.degree., but
depending on inter alia the width of the information areas, to a
phase depth of for example 115.degree.. For a structure with
oblique walls the concept of phase difference can in fact no longer
be used.
According to the method of recording now preferred a photoresist
layer deposited on a substrate is exposed to an optical write beam
whose intensity is switched between a high and a low level in
accordance with the information to be written. After exposure the
photoresist is developed, pits being formed at the locations which
have been exposed to a high intensity. From a so-called "master"
thus obtained a large number of copies can be manufactured with the
aid of techniques known from the manufacture of audio discs. When
information is recorded in a photoresist layer whose thickness is
substantially greater than the depth of the information areas in
the final information carrier, information areas with the said
small phase depth can only be realized with large angles of
inclination of the walls.
It is an object of the present invention to provide a record
carrier whose information-area walls have a substantial angle of
inclination, which can be read in an optimum manner with the aid of
the radiation sources which are used most frequently in practice,
namely a helium-neon gas laser and an AlGaAs diode laser.
The record carrier in accordance with the invention is
characterized in that the cross-section, transverse to the track
direction, of the information areas is substantially V-shaped, that
the phase depth of the information areas has one value between
100.degree. and 125.degree., and that the angle of inclination
between the walls of the information areas and the normal to the
record carrier is substantially constant and has a value between
65.degree. and 85.degree..
The information areas may consist of pits pressed into the record
carrier surface or of hills projecting from the record carrier
surface.
In theory the information areas may have a V-shape with acute
angles. However, in practice the information area will be more
gradual pits or hills. These information areas have no flat bottom
or top, as the information areas of the record carrier described in
the article: "Simplified diffraction theory of the video disk." The
phase depth of the information areas in the record carrier in
accordance with the invention is mainly determined by the wall
steepness of said areas. For angles of inclination of the order of
magnitude of 65.degree. to 85.degree. the phase depth of the
information areas is preferably 110.degree.. For a specific angle
of inclination and a specific width of the information areas,
measured transversely to the track direction, the average
geometrical depth of a pit or the average geometrical height of a
hill has a fixed value. The optical depth corresponding to this
average geometrical depth is always smaller than 1/8.lambda.eff,
the optical depth being for example 1/10 .lambda.eff. The optimum
value, within the said limits, of the angle of inclination depends
on the read beam used, specifically the wavelength of said beam in
relation to the width of the information areas and, to a smaller
extent, on the state of polarization of said beam.
A record carrier in accordance with the invention which is adapted
to be read with a read beam produced by a helium-neon gas laser and
having a wavelength of approximately 633 nm, in which record
carrier the width, transverse to the track direction, of the
information areas is approximately 625 nm, is characterized in that
the angle of inclination of the information areas is approximately
78.degree..
A record carrier in accordance with the invention is adapted to be
read with a read beam produced by an AlGaAs diode laser with a
wavelength in the range of 780-860 nm, and with a direction of
polarization parallel to the track direction, in which record
carrier the width, transverse to the track direction, of the
information areas is approximately 625 nm, is characterized in that
the angle of inclination of the information areas is approximately
73.degree..
If a read beam is used whose wavelength lies between 633 nm and 780
nm, the optimum value of the angle of inclination lies between
73.degree. and 78.degree.. For a read beam with a wavelength
smaller than 633 nm the optimum angle of inclination has a value
between 78.degree. and 85.degree..
The invention is particularly suitable for use in a record carrier
in which apart from shallow phase structures, to be read in
push-pull, contains also deeper phase structures to be read by the
so-called "central aperture" method. For the central aperture
method the information is read by detecting the sum of all the
radiation intensity passing through the exit pupil of the objective
system. When a record carrier is to be provided with both a deeper
and a more shallow phase structure, the more shallow phase
structure can be realized almost only with large angles of
inclination by means of the write method now preferred. Two types
of information areas in one record carrier may for example be used
in order to obtain a high information density, as is described in
U.S. patent application Ser. No. 925,433, filed July 18, 1978. If
in such a record carrier use is made of the concept underlying the
invention, said record carrier is characterized in that between
first information tracks containing information areas with a phase
depth between 100.degree. and 110.degree. second information tracks
are formed which contain information areas whose phase depth is
approximately 180.degree..
The present invention cannot only be used in a record carrier which
is completely provided with information but also in a record
carrier in which information can be written by the user himself. In
such a record carrier the information is address information
contained in so-called sector addresses, each track containing a
specific number of such addresses. The sector addresses occupy only
a small part of the tracks. The track portions between the sector
addresses are of an inscribable material, for example a thin
metallic layer, in which the user can then record information with
the aid of for example a laser beam, for example by locally melting
the metal. A sector address contains address information of the
associated inscribable track portion in the form of address areas
which are spaced from each other by intermediate areas. According
to the invention the address areas have a substantially V-shaped
cross-section, a phase depth between 100.degree. and 125.degree.,
and an angle of inclination between 65.degree. and 85.degree..
The invention will now be described in more detail with reference
to the drawing. In this drawing:
FIG. 1 shows a part of an information structure of a round
disc-shaped record carrier,
FIG. 2 shows a part of a tangential cross-section of a record
carrier in accordance with the invention
FIG. 3 shows a part of a radial cross-section of this record
carrier,
FIG. 4 shows a known apparatus for reading the record carrier,
FIG. 5 shows the variation of the amplitude of the information
signal as a function of the phase depth,
FIG. 6 shows a composite detection system and the block diagram of
an associated processing circuit by means of which, in addition to
an information signal, a positional error signal can be
obtained,
FIG. 7 shows a part of a record carrier in accordance with the
invention, containing information tracks with a greater phase depth
and information tracks with a smaller phase depth,
FIG. 8 shows a part of a radial cross-section of this record
carrier,
FIG. 9 shows a part of a tangential cross-section or a record
carrier in accordance with the invention which within a track
comprising track portions with a greater phase depth and track
portions with a smaller phase depth,
FIG. 10 shows a record carrier in accordance with the invention in
which information can be recorded by a user.
As is shown in FIG. 1 the information structure comprises a number
of information areas 2, which are arranged in accordance with
information tracks 3. In the track direction or tangential
direction t the information areas are spaced from each other by
intermediate areas 4. In the radial direction r the information
areas are spaced from each other by lands 5. The information areas
may comprise pits pressed into the record carrier surface or hills
projecting from the record carrier surface. In principle the depth
of the pits, or the height of the hills is constant, and so is the
width of the information areas and intermediate areas at the level
of the plane of the lands. The said distance and the said width are
not determined by the information which is stored in the
structure.
The information to be conveyed by means of the record carrier is
contained in the variation of the structure of areas in the
tangential direction only. If a color television program is stored
in the record carrier, the luminance signal may be coded in the
variation of the spatial frequency of the information areas 2 and
the chrominance and audio signal in the variation of the lengths of
the areas 2. Instead of a television program the record carrier may
contain an audio program. The information may also be digital
information. In that case a specific combination of information
areas 2 and intermediate areas 4 represents a specific combination
of digital ones and zeros.
Such a record carrier with a radiation reflecting information
structure can be read with an apparatus which is schematically
represented in FIG. 4. A monochromatic and linearly polarized beam
11 emitted by a gas laser 10, for example a helium-neon laser, is
reflected to an objective system 14 by a mirror 13. In the path of
the radiation beam 11 there is arranged an auxiliary lens 12 which
ensures that the pupil of the objective system 14 is filled. On the
information structure a diffraction-limited read spot V is then
formed. The information structure is schematically represented by
the tracks 3; i.e. the record carrier is shown in radial
cross-section.
The information structure may be disposed on the record carrier
side which faces the laser. However, preferably, as is shown in
FIG. 4, the information structure is disposed on the record carrier
side which faces away from the laser, so that the record carrier is
read through the transparent substrate 8. The advantage of this is
that the information structure is protected against fingerprints,
dust particles, and scratches.
The read beam 11 is reflected by the information structure and as
the record carrier is rotated by means of a platter 16 which is
driven by a motor 15, it is modulated in accordance with the
sequence of the information areas and the intermediate areas 4 in a
track being read. The modulated read beam again passes through the
objective system 14 and is reflected by the mirror 13. In order to
separate the modulated read beam from the unmodulated read beam the
radiation path preferably inculdes a polarization-sensitive
splitter prism 17 and a .lambda..sub.o /4 plate 18, where
.lambda..sub.o represents the wavelength of the read beam in free
space. The beam 11 is transmitted by the prism 17 to the
.lambda..sub.o /4 plate 18, which converts the linearly polarized
radiation into circularly polarized radiation which is incident on
the information structure. The reflected read beam again passes
through the .lambda..sub.o /4 plate 18, the circularly polarized
radiation beam being converted into linearly polarized radiation
whose plane of polarization is rotated through 90.degree. relative
to the radiation emitted by the laser 10. Thus, upon the second
passage the read beam will be reflected by the prism 17, namely to
a radiation-sensitive detection system 19.
For reading the information thee detection system should comprise
two detectors which are arranged behind each other in the effective
track direction. The output signals of the detector are subtracted
from each other in a circuit which is schematically represented by
the block 20 in FIG. 4. The output signal S.sub.i is determined by
the sequency of information areas and intermediate areas in the
track portion being read instantaneously. After decoding, this
signal is displayed on a television set if the record carrier
contains a television program, or is reproduced with known audio
equipment if an audio program has been recorded on a record
carrier.
As stated in the previously mentioned article: "Simplified
diffraction theory of the video disk," the phase difference between
a beam portion coming from an information area and a beam portion
coming from the vicinity of said area should be 90.degree. for an
optimum signal S.sub.i in the case that the walls of the
information areas are perpendicular. The phase difference of
90.degree. corresponds to a phase depth of for example
115.degree..
In a record carrier in accordance with the invention the
information areas have oblique walls, as is shown in FIGS. 2 and 3.
The concept of phase difference can then no longer be used and the
concept of phase depth is to be adopted. FIG. 2 shows a small part
of the record carrier in accordance with the invention in
tangential cross-section taken on the line II--II' in FIG. 1, while
FIG. 3 shows a small part of this record carrier in radial
cross-section, taken on the line III--III' in FIG. 1. During
reading the record carrier is illuminated from the underside, the
transparent substrate 8 being used as an optical protective layer.
The information structure may be covered with a layer 6 of a
reflecting material, such as silver or aluminium or titanium. Onto
the layer 6 another protective layer 7 may be deposited, which
protects the information structure against mechanical damage such
as scratches. FIG. 3 furthermore shows the angle of inclination
.theta. of the radial walls 9 of the information areas, i.e. of the
transitions from information areas to lands. The angle of
inclination .theta..sub.2 of the tangential walls 9' of the
information areas, i.e. of the transitions from information areas
to intermediate areas, shown in FIG. 2 is of the same order of
magnitude as .theta..sub.1. As in general the length of the
information areas is greater than their width, these areas have
straight portions in the cross-section of FIG. 2.
Calculations and experiments conducted by the applicant have
demonstrated that the write process and the copying process are
reproducible in an optimum manner if the angle of inclination
.theta..sub.1 has a value between 65.degree. and 85.degree..
Furthermore it has been found that, within these limits for the
angle of inclination .theta..sub.1, the optimum phase depth is
approximately 110.degree.. FIG. 5 represents the variation of the
amplitude A.sub.S.sbsb.i of the information signal S.sub.i as a
function of the phase depth .PSI.. For a phase depth
.PSI.=180.degree. the energy distribution within the exit pupil of
the objective system 14 is symmetrical, so that the difference
signal from the detectors is zero. A phase depth .PSI.=90.degree.
means that the information structure is very shallow. The amplitude
of the spectral first orders is then approximately zero.
Consequently the amplitude A.sub.S.sbsb.i is also zero for
.PSI.=90.degree.. FIG. 5 also reveals that the phase depth
.PSI.=110.degree. is the optimum value, but that also at deviating
values an acceptable information signal S.sub.i can be obtained.
For .PSI.=100.degree. and .PSI.=125.degree. the amplitude of the
signal S.sub.i is still approximately 80.degree. of the optimum
value, so that information areas with a phase depth of 100.degree.
to 125.degree. can be read reasonably.
The phase depths plotted along the horizontal axis of FIG. 5 result
from different geometries of the information areas, specifically
from different values of the wall steepness of these areas. The
wall steepness is determined by the intensity of the write beam
which is used and by the developing process.
The Applicant has come to recognize that apart from by the angle of
inclination .theta..sub.1 of the walls of these areas, the observed
phase depth of the information areas is determined by:
the effective wavelength of the read beam in relation to the
maximum width of the information areas, and
the state of polarization of the read beam. The effective
wavelength is the wavelength near the information structure and
outside the radiation reflecting layer. In the case shown in FIGS.
1, 2 and 3 the effective wavelength is equal to the wavelength in
free space divided by the refractive index (N) of the substrate
8.
In the case of V-shaped information areas the wall steepness
determines the effective depth of said areas.
According as the wavelength of the read beam increases, the
effective depth and thus the wall steepness of the information
areas should be increased in order to obtain a specific phase
depth.
For read out with a HeNe read beam the optimum phase depth
.PSI.=110.degree. is obtained for an angle of inclination of
78.degree. and for read out with an AlGaAs read beam for an angle
of inclination of 73.degree.. If the average spatial frequency of
the information areas varies over the record carrier, for example
if on a record carrier with a television program one television
picture per revolution is stored, the wall steepness may be
increased at greater average spatial frequency of the information
areas in order to obtain an optimum information signal over the
entire record carrier.
In general can be stated that when a perpendicularly polarized read
beam is used an elongate pit or hill generally appears to be deeper
or higher than when a parallel polarized read beam is used. A
perpendicularly polarized or parallel polarized read beam is to be
understood to mean a read beam whose electrical vector, the
E-vector, is respectively perpendicular or parallel to the
longitudinal direction of the pits or hills.
When a He-Ne laser source is used and when an AlGaAs diode laser is
used the said polarization effect occurs. When reading by means of
a HeNe laser, as is described with reference to FIG. 4, a
circularly polarized read beam is incident on the information
structure. A diode laser emits linearly polarized radiation. When a
diode laser is used in a read apparatus use can be made of the
so-called "feedback," the diode laser being employed as detector.
In that case no polarization means need be included in the
radiation path, as in the apparatus in accordance with FIG. 4, and
the information structure is scanned with linearly polarized
radiation. If the read beam is perpendicularly polarized, the
information areas should have a greater angle of inclination in
order to obtain a phase depth of 110.degree., than if the read beam
is parallel or circularly polarized. In the case of a
perpendicularly polarized read beam the observed phase depth
increases more rapidly at decreasing angle of inclination than in
the case of a parallel or circularly polarized read beam.
Preferably, reading is effected with a parallel polarized beam,
because the angle of inclination is then less critical.
When reading the record carrier care must be taken that the center
of the read spot always remains positioned on the center of a track
to be read. For this purpose a positional error signal should be
generated, which provides an indication about the magnitude and
direction of a possible deviation from the center of the read spot
relative to the track center. This positional error signal can be
obtained with the aid of two detectors which, in the direction
transverse to the effective track direction, are offset relative to
each other. The output signals of these detectors are applied to a
subtractor circuit, 21 in FIG. 4. The output signal S.sub.r of this
circuit then constitutes the positional error signal. This signal
can be processed in the circuit 22, known per se, into a control
signal for correcting the position of the read spot, for example by
tilting the mirror 13 about the axis 25.
The record carrier in accordance with the invention whose
information structure has been optimized for information read out
is also correctly dimensioned for generating an optimum positional
error signal. This is because the positional error signal is also
obtained by push-pull reading, but now in a direction transverse to
the track direction. In the previously mentioned article:
"Simplified diffraction theory of the video disk" it has been
demonstrated that in the case of push-pull reading of information
areas with perpendicular walls both the information signal and the
positional error signal are optimum for a phase difference of
90.degree.. In a similar way both the information signal and the
positional error signal are optimum for a phase depth of
.PSI.=110.degree. when information areas with oblique walls are
read in push-pull.
A known detection system, by means of which both an information
signal and a positional error signal can be obtained, is shown in
FIG. 6. The detection system comprises four detectors 25, 26, 27
and 28, which are disposed in four different quadrants of an
imaginary X-Y coordinate system. The X-axis and the Y-axis are
effectively parallel to the track direction t and the radial
direction r respectively (compare FIG. 1). The output signals of
the detectors 25 and 26 are applied to the summing device 29, and
the output signals of the detectors 27 and 28 to a summing device
30. The signals supplied by these summing devices are applied to a
differential amplifier 21, on whose output the positional error
signal S.sub.r is available. The information is read by applying
the output signals of the summing devices 31 and 32, whose inputs
are connected to the detectors 25, 28, and 26, 27 respectively, to
a differential amplifier 20. The information signal S.sub.i is
available on the output of this amplifier. It has already been
proposed in the previous patent application Ser. No. 925,433, filed
July 18, 1978 in order to increase the information density, to
insert second information tracks whose information areas have a
smaller phase depth between first information tracks whose
information areas have a phase depth of approximately 180.degree..
By reading the first information tracks in the central aperture
mode and the second information tracks in the push-pull mode,
little cross-talk between the two types of information tracks will
occur during reading. Furthermore, consecutive track portions
within one tack may also be distinguished from each other in that a
first track portion contains information areas having a phase depth
of approximately 180.degree. and a subsequent track portion
information areas with a smaller phase depth.
In accordance with the invention the information areas with the
smaller phase depth may have a V-shape with an angle of inclination
between 65.degree. and 85.degree..
FIG. 7 shows a part of such a record carrier. In addition to
information tracks 3 comprising information areas 2 of smaller
phase depth there are information tracks 3' comprising information
areas 2' of greater phase depth. The distance between the track 3
and the track 3' is smaller than the distance between two tracks 3
in FIG. 1.
FIG. 8 shows a radial cross-section taken on the line VIII--VIII'
in FIG. 7. FIG. 8 partly corresponds to FIG. 3. In FIG. 8
information areas 2' are situated at the locations of the lands 5
in FIG. 3. These information areas preferably also have oblique
walls whose angle of inclination .theta..sub.3 lies between
30.degree. and 60.degree.. The geometrical structure of the
information areas 2' has been described elsewhere, namely in
another patent application Ser. No. 972,754, filed Dec. 26,
1978.
At a phase depth of 110.degree. of the information areas 2 an
optimum information signal is obtained from the areas in the case
of push-pull reading. However, a phase depth of 110.degree. also
results in an appreciable signal in the case of central aperture
reading, which is use for reading in the information areas 2'.
Preferably, the phase depth of the information areas 2 is selected
near 100.degree.. The push-pull signal from the information areas 2
is then still large, while in the case of central aperture reading
of the information areas 2' the information areas 2 are hardly
detected.
FIG. 9 shows a tangential cross-section of a record carrier which
within a track comprises track portions of a smaller phase depth
alternating with track portions of a larger phase depth, the
cross-section representing a transition from a first track portion
to a second track portion. After the foregoing this figure is
self-explanatory.
In for example the previous application Ser. No. 925,229, filed
July 18, 1978, it has already been proposed to employ an optical
record carrier as a storage medium for information other than video
information, especially as a storage medium in which information
can be recorded by the user himself. Examples of this are
information supplied by an (office) computer or radiograms made in
a hospital. For this purpose the user is supplied with a record
carrier which is provided with a so-called servo-track of for
example spiral shape, which extends over the entire record carrier
surface.
During the recording of the information by the user the radial
position of the write spot relative to the servo track is detected
and corrected with the aid of an opto-electronic servo system, so
that the information is written with high accuracy in a spiral
track of constant pitch. The servo track is divided into a
multitude of sectors, for example 128 per revolution. FIG. 10 shows
a plan view of such a record carrier 50. The servo track is
designated 51 and the sectors 52. Each sector comprises a track
portion 54, in which the information may be recorded, and a sector
address 53, in which inter alia the address of the associated track
portion 54 is encoded in address areas in for example digital form.
The address areas are spaced from each other by intermediate areas
in the track direction. The address areas may comprise pits pressed
into the record carrier surface or hills projecting from said
surface.
In accordance with the invention the address areas consist of pits
or hills with oblique walls having an angle of inclination between
65.degree. and 85.degree. and these address areas have a phase
depth between 100.degree. and 125.degree. in a similar way as
described in the foregoing for the information areas in a record
carrier with a video program. A tangential cross-section of the
sector addresses is then as shown in FIG. 2. Preferably, the sector
addresses of all tracks are situated within the same circular
sectors. In that case a radial cross-section through the address
areas will be as shown in FIG. 3.
The "blank" track portions 54 may comprise continuous grooves on
which a layer of a reflecting material is deposited which, if
exposed to suitable radiation, is subject to an optically
detectable change. The layer for example consists of bismuth in
which information areas can be formed by melting.
The "blank" track portions may consist of V-shaped grooves. In
order to enable an optimum tracking signal to be obtained from
these grooves by push-pull reading during recording, these grooves,
as is apparent from the foregoing, should have a phase depth which
is approximately 110.degree.. When a record carrier inscribed by
the user i.e. a record carrier in which pits have been melted in
the V-shaped grooves, is read in the central aperture mode the
groove portions between the pits will still produce a small signal,
if these groove portions have a phase depth of 110.degree..
Therefore, the phase depth of the blank grooves is preferably
100.degree., so that during central aperture reading of the
inscribed record carrier these grooves are hardly detected
anymore.
The invention has been described on the basis of a round
disc-shaped record carrier. However, the invention may also be used
for other record carriers, such as tapelike or cylindrical record
carriers, which contain information in a phase structure.
* * * * *